The first thing you need to do is ensure you have a recent version of mamp, as the zendframework requires the latest stable build of php, it will also help to download the latest zendAMF and replace the one that Flashbuilder installed (you just over write the folder with the new code)

Some people have had issues on macs where the zend pathing wasn't working /recognised, there is info on this spread through http://forums.adobe.com/community/flex/flex4beta , searching on 'zend' in the forum will yield several thoughts if you have problems after trying the above.